The present invention relates to circuitry used along with a linear variable differential transducer (LVDT) to sense the position of some element. Specifically the invention provides circuitry and a method to sense the position of an element which is efficient, accurate, and uses small amounts of power.
To sense physical position of different devices it is common place to utilize a linear variable differential transducer (LVDT), which typically has a primary coil, a secondary coil and a core member that magnetically couples the primary coil to the secondary coil. The amount of magnetic coupling is dependent upon the position of the core within the transducer. By measuring the amount of magnetic coupling between the primary coil and the secondary coil, the position of the core member can be determined. Similarly, when the core member is attached to some element, the position of that element can also be determined by measuring the amount of magnetic coupling between the primary and secondary coils.
To measure the amount of magnetic coupling between the primary coil and the secondary coil, the primary coil is excited by an AC signal which creates an induced AC signal on the secondary coil. The induced AC signal on the secondary coil is then measured and compared with the AC signal used to excite the primary coil. The ratio of the induced AC signal at the secondary coil to the excitation signal is indicative of the amount of magnetic coupling provided by the core member. Since the amount of magnetic coupling provided by the core member is related to the position of the core member, the ratio indicating magnetic coupling is also indicative of the core member position.
Constant excitation of the primary coil by an AC signal creates problems. Specifically, a high amount of power is used due to constant excitation of the primary coil. Additionally, problems with cross modulation signals can become very severe when AC signals are utilized.
Recently square wave pulses have been used to excite the LVDT primary, along with appropriate time delays to eliminate the problems of noise associated with AC excitation. In this method of LVDT sampling, a square wave is used to drive the transformer primary, and a delay time is allowed to pass before the signal at the secondary is measured. This delay allows the signal at the secondary to stabilize sampling.
Typically, sampling of the secondary is accomplished by sample and hold circuits attached to each secondary coil. While this is an effective way to sample the secondary signal, it involves a large amount of circuitry.